Off-axis collimation optics

ABSTRACT

A light funnel collimator has a central lens surface and a back reflecting surface, shaped to provide a wider back-ground beam and a narrower hotspot beam within but off-center of the wider beam. One of the beams is on-axis of the collimator, and the other beam is off-axis. The reflector is at least partly asymmetrical relative to the axis, and provides or contributes to the off-axis beam.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationNo. 61/298,030, filed Jan. 25, 2010 by Dross et al. for “Off-axiscollimation optics.”

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) are widely available, inexpensive, andefficient light sources. For uses such as sport headlamps, one or twostate of the art LEDs provide adequate light. While simple lightdistributions of rotational symmetry are sufficient for low qualityproducts or less demanding uses, more complex light distributions arebeing employed for better vision when walking, running, or cycling witha headlamp. It is beneficial to produce a relatively narrow hotspot oftypically 10° full width at half maximum (FHWM) so to illuminate objectsfar away from the user, while a lower level intensity background lightis needed to provide lighting of the ground close to the user. Such abackground light is not needed upwards from the hotspot so that abackground beam that is tilted relative to the hotspot beam isbeneficial.

A collimator configuration as seen in FIG. 1 is used in a currentheadlamp product made by Silva Sweden AB that produces a wide beamoff-axis background light around a narrow intense on-axis hotspot. Thelens 100 is of a type herein called a “photon funnel” that has a central“collimator cavity” containing the light source. The wall of the cavityhas a front or center lens 103 and a side or peripheral cavity surface.In a center section of the photon funnel, light passes by refractionthrough the center lens 103 and an exit surface (which in FIG. 1 is partof a front surface 104), while the majority of the light passes throughthe cavity side surface by refraction, is reflected by total internalreflection (TIR) at a back surface 102, and exits through the frontsurface 104 by refraction.

In the Silva product, the center lens 103 of lens 100 is a rotationallysymmetric surface that has its rotational axis tilted with respect tothe light source axis to provide an off-axis background light whilesurface 102 collimates the majority of the light from the LED to form anarrow hot spot. This architecture works well, if the amount of lightthat is needed for the background illumination is roughly one third ofthe full light emitted by a Lambertian LED, as this is the typicalamount of light collected by the center lens of a conventional photonfunnel. If more or less light is wanted in the off-axis beam, thisconfiguration cannot be used. Moreover the center lens provides arelatively wide beam by nature of the lens 103, so that if a narrowoff-axis beam is wanted, the center lens cannot provide such beam.

In all of the described embodiments, the cavity side surface is asurface of rotation about a center axis, and the light source is an LEDchip centered on and coaxial with the center axis of the cavity. Atypical LED chip is flat, and is a Lambertian emitter with its emissionsymmetrical about an axis perpendicular to the flat chip. The LED chipthus typically has a well-defined central axis. In the presentspecification, the terms “on-axis” and “off-axis” are used here withrespect to the common center axis of the collimator cavity and the LEDchip. In all of the embodiments, one of the hotspot beam and thebackground beam is directed along the center axis, and the other beam isdirected along a second axis, referred to as a “tilted axis,” divergingfrom the center axis. In all of the embodiments, the exit surface of theoptics is flat, and the surface normal of the exit surface coincideswith the cavity center axis. However, exit surfaces of other shapes andorientations can be implemented.

The head lamp itself often provides means to adjust the direction oflight emission of the entire lamp, so that the narrow beam can beadjusted for far vision while the wide beam will provide near vision.Thus, as will be shown below with reference to FIG. 4, the samefunctionality as in the Silva lamp can be achieved by the “dual” case inwhich a tilted center lens provides a hotspot beam along the tiltedaxis, and an on-axis reflector provides an on-axis background beam.However, the simple dual configuration will then typically directtwo-thirds of the light into the background beam and one-third into thehotspot beam, which may not be optimal.

Other applications besides sport headlamps of partially or fullyoff-axis LED collimators would be in architectural lighting to createcertain lighting effects, such as illuminating a wall from a lightingfixture that is oriented parallel to the wall, in street lighting, andmany other applications.

SUMMARY OF THE INVENTION

The optical approach explained in detail below does not rely solely onthe center lens of a photon funnel to provide off-axis illumination.Using the TIR reflective back surface of a photon funnel for off-axisillumination has several advantages, among them: that much more fluximpinges upon this surface; and that by the nature of reflection,modifications of the back surface make much larger off-axis beam tiltangle possible than with a single refraction at the center lens. In allof the embodiments described below, the optical designs are modifiedrotational designs. The rotational designs are obtained with commonmethods, either with point source approximation numerical or analyticmethods or with extended source optimization using common iterativenumerical methods. The starting point design can be a narrow-beamon-axis collimator, or part of the surfaces can be calculated to providea wider on-axis beam. In a subsequent step some optical surfaces aremodified to deviate from the rotational symmetry. All other surfaces maybe left unchanged, including the so-called cavity surfaces (thecircumferential wall of the central cavity, through which light entersthe photon funnel dielectric on a path towards the back reflectivesurface) and the front (exit-) surface of the dielectric. In thefollowing detailed description and drawings, examples of photon funnelswith a fully or partially modified TIR back surface are described andshown. The center lens may or may not also be modified, to provideadditional on and off axis illumination, and all combinations ofmodified center lenses and modified mirrors are possible. The backsurface may be modified so that a modified section of the reflectorsurface provides off-axis light while an unmodified section provideson-axis light. Both beam spreads, the angle of tilt between the on andoff-axis portions and their intensity patterns and levels can becontrolled. When modifying both the center lens and back surfacecompletely, all light can be sent off-axis, either in a single beam orin two (or more) differently tilted beams.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 shows a photon funnel of the prior art.

FIG. 2 shows the first preferred embodiment of a photon funnel with aback surface to provide off-axis narrow beam illumination and on-axiswide background light provided by the center lens.

FIG. 3A shows an intensity distribution in the horizontal and verticaldirection, of a photon funnel with an off-axis hotspot and an on-axislow intensity background intensity.

FIG. 3B shows the same radiation pattern as a 2 dimensionaldistribution.

FIG. 4 shows a photon funnel that provides a narrow off-axis hotspotfrom the center lens and a wide on-axis background illumination from theback surface.

FIG. 5 shows a photon funnel that provides a narrow on-axis hotspot fromthe center lens and a wide off-axis background illumination from theback surface.

FIG. 6 shows a photon funnel that provides a narrow off-axis hotspotfrom a top section of the back surface.

FIG. 7 shows a photon funnel that provides wide beam on-axis backgroundillumination from a bottom section of the back surface.

FIG. 8 shows a three dimensional view of a photon funnel as constructedaccording to FIG. 6 and/or FIG. 7.

FIG. 9 shows the 2D wavefront method to calculate meridiancross-sections for an improved embodiment for off-axis illumination fromthe back surface.

FIG. 10 shows a three dimensional view of the back surface constructedaccording to FIG. 9.

FIG. 11 shows the 3D wavefront method to calculate a freeform backsurface.

FIG. 11A shows prefixed photon funnel surfaces from a standardrotational design.

FIG. 11B shows the 3D wavefronts from the source and target used toderive the freeform back surface.

FIG. 11C shows the exit surface and the calculated freeform backsurface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription of the invention and accompanying drawings, which set forthillustrative embodiments in which the principles of the invention areutilized.

In the following drawings all architectures are described in moredetail:

Referring to FIG. 2, a lens 200 represents a photon funnel thatcollimates the light from a source 201 into an off-axis hot spot andalso creates a low-intensity background illumination. The photon funnel200 comprises a dielectric lens with a central cavity. Lines 212 in FIG.2 are cross sections of a conical surface of rotation with respect tothe source axis 203, forming the side wall of the cavity. Surface 202forms a central lens, closing off the front end of the cavity, and is arotationally symmetric surface around axis 203. Lens surface 202 isshaped so that rays such as 204 and 205 that are refracted at lenssurface 202 and at flat exit surface 211 produce a uniform backgroundillumination.

In general, references to a “surface of rotation,” “rotationallysymmetric surface,” or similar indicate that the surface can begenerated by rotation of a generator line, which may be straight,curved, or of a more complicated shape, about an axis, but do notrequire that the physical surface forms a complete annulus about theaxis, nor that the axial ends of the physical surface form circlescoaxial with the axis. As will be explained below, many of theembodiments described can be formed with surfaces that may be either asingle annulus or two or more distinct arcs, and in many of theembodiments an axial boundary may be a junction (with or without anoptically inactive step) between surfaces that are rotationallysymmetric about different axes.

The back of photon funnel 200 is formed by a single continuous surface,represented by curves 206 and 207 in FIG. 2, that is rotationallysymmetric with respect to an axis 208 that is tilted relative to thesource axis 203. Thus, the curves 206, 207 are not identical, becausethey represent different parts of the surface of rotation, with curve207 starting closer to the (tilted) axis 208. Reflective surface 206 isshaped so that rays such as 209 and 210 that are refracted at sidesurface 212 undergo total internal reflection at surface 206 and thenexit through surface 211, forming an off-axis narrow beam pattern.

FIG. 3A shows two-dimensional horizontal and vertical sections through atypical intensity pattern produced by a partially off-axis collimatorsuch as that shown in FIG. 2. The background emitted by the center lensis located with its center on-axis. The light has a FWHM of about 45deg. The hotspot, emitted by the back surface 206, is located off-axis.In FIG. 3B, a two-dimensional intensity pattern is shown to illustratethe same radiation pattern. The scales on the horizontal axis in FIG. 3Aand the vertical and horizontal axes in FIG. 3B represent angles indegrees from the center axis 203. The contour lines in FIG. 3B and thevertical axis in FIG. 3A are normalized intensity levels.

Lens 400 in FIG. 4 is an optical device that collimates the light from asource 401 into an off-axis narrow hot spot and also creates alow-intensity background of rotational symmetry. A central cavity isbounded by a conical surface of rotation with respect to the source axis403, seen in cross section as lines 412 and 413. Lens surface 402 is arotationally symmetric surface around tilted axis 408. As a result, theline where lens surface 402 meets conical surface 412, 413 is tilted,shown by the greater length of section line 413 than section line 412.Rays such as 404 and 405 that are refracted at lens surface 402 andplanar exit surface 411 will form the narrow off-axis beam pattern.Surface 406 forming the back of the light funnel 400 is rotationallysymmetric around (non-tilted) light-source axis 403. The cross-sectionof surface 406 is calculated to provide a wide background pattern. Rayssuch as 409 and 410 that are refracted at surface 412, 413 undergo totalinternal reflection at the back surface, represented by curves 406 and407, and then exit through planar surface 411.

Lens 500 in FIG. 5 is an optical device that collimates the light fromsource 501 into an on-axis hot spot and also creates a low-intensityoff-axis background illumination. A central cavity is bounded by conicalsurface 512, which is a surface of rotation with respect to the sourceaxis 503, and by lens surface 502, which is a rotationally symmetricsurface around source axis 503. Rays such as 504 and 505 that arerefracted at lens surface 502 and planar exit surface 511 will form thenarrow on-axis beam pattern. The back surface, represented by curves 506and 507, is a rotationally symmetric surface around tilted axis 508. Thecross-section of surface 506, 507 is calculated to provide an off-axiswide background pattern. Rays such as 509 and 510 that are refracted atconical surface 512 undergo total internal reflection at surface 506,507 and then exit through planar surface 511.

Lens 600 in FIGS. 6 and 7 is an optical device that collimates the lightfrom source 601 into an off-axis hot spot and also creates an on-axislow-intensity background. By splitting the back surface into sections,the amount of light directed into on-axis and off-axis beams can beadjusted to the application. Side surface 609 of the central cavity is aconical surface of rotation with respect to the source axis 605. Surface602, which forms the front part of the TIR back surface of light funnel600, is rotationally symmetric around tilted axis 604. Tilted reflectorsurface 602 may be seen as being obtained by tilting a surface 611 thatis rotationally symmetric around source axis 605 around the center ofsource 601. Rays such as 606 and 607 that are refracted at cavity sidesurface 609, then undergo total internal reflection at tilted reflectivesurface 602, and then exit passing through flat front surface 608 willform the off-axis hot spot.

Lens surface 615 and reflective surface 613, which forms the rear partof the TIR back surface of light funnel 600, is rotationally symmetricaround source axis 605. Rays 705 (see FIG. 7) that are refracted atcavity side surface 609, then undergo total internal reflection aton-axis reflective surface 613, and then exit passing through flat frontsurface 608 contribute to the on-axis background illumination. Rays 704that are refracted at the front lens surface 615 of the cavity and thenat the flat front surface 608 also contribute to the on-axis backgroundillumination.

As may be seen from FIGS. 6 and 7, the relative intensities of thehot-spot and background beams may easily be set by choosing the positionof the transition between the front and rear sections 602 and 613 of thereflector surface.

FIG. 8 provides a three dimensional view of a photon funnel lens 800,with source axis 805, which may be similar to the lens 600 shown inFIGS. 6 and 7 described above.

Various methods of construction can be used to obtain a back surface ofa photon funnel to provide off-axis illumination or non rotationallysymmetric illumination:

1. The whole or a section of a rotationally symmetrical collimating backsurface such as surface 611 is tilted (FIG. 6) around the source centerby an angle equal to ArcSin((Sinθ)/n), where θ is the desired off axisangle of the center of the illumination pattern and n is the index ofrefraction of the dielectric used. Because of the refraction of thelight at the cavity wall 609, the system does not behave like a lightsource in an air filled reflector, so that for large pitch angles aroundthe source center no optically “correct” surface for the illuminationtask can be obtained. For small angles of pattern center shift (up toapproximately 20 deg) this method works sufficiently well. This solutionprovides an asymmetric exit aperture that is off centered from theoptical axis 605.

2. In FIG. 9 a more precise procedure is illustrated. A meridiancross-section 902 is calculated as a generalized Cartesian oval derivedfrom off-axis wavefront 909 (outgoing wavefront after refraction at theexit surface 908) and source wavefront 911 (source wavefront afterrefraction at cavity wall 910). The source is treated like a pointsource, so that wavefront 911 can be represented as a sphericalwavefront. The meridian cross-section 903 is calculated similarly fromwavefronts 912 and 913. Both cross-sections are rotated around a tiltedaxis 904 and result in surfaces 1001 and 1002 in FIG. 10. In FIG. 10,only the back surfaces of the photon funnel are shown. The surfaces 1001and 1002 do not necessarily intersect. The left half (as seen in FIGS. 9and 10) of 1001 and the right half of 1002 are cut and connected byoptically inactive surfaces 1003 and 1004.

3. For large off-axis angles or for a more complex off-axis or nonrotationally symmetrical radiation pattern, a new back surface can bederived: In FIG. 11A, a source 1104 and a cavity, consisting of cylinder1103 and cavity lens 1102, are shown. Exit surface 1101 is a flatsurface. The back surface is derived as follows. An outgoing wavefront1105 (FIG. 11B) is chosen that contains the information of what off-axisradiation pattern is to be obtained. The outgoing wavefront is in thiscase a cylindrical surface, although any other suitably well-behavedwavefront with or without any symmetry can be used. However it must bepossible to propagate the wavefront free of caustics throughout thespace in which the back surface is being created. A centered cylindricalwavefront 1105 would provide an extended oval beam pattern, centered atthe source axis. The outgoing wavefront must be traced back through theexit surface. The source wavefront, in the point source approximation,is a spherical wavefront that, when propagated and refracted at thecavity, becomes an aspheric rotationally symmetric surface 1106. Ageneralized Cartesian surface that couples wavefronts 1105 and 1106 canbe numerically calculated and is shown as freeform surface 1107 (FIG.11C), which will be the back reflecting surface for the photon funnel(cavity and lens are not shown in FIG. 11C for simplicity).

Although specific embodiments have been described, the person skilled inthe art will understand how features of different embodiments may becombined, and how features may be substituted or modified, withoutdeparting from the scope of the claimed invention.

For example, although the reflectors 206, 207, 406, 407, 506, 507, 602have each been described as a single rotationally symmetrical surface,any of them may be designed by any of the methods described withreference to any of FIGS. 9 and 11, and may therefore be two (or more)surfaces separated by an axial cut line, as in FIG. 10, or anasymmetrical surface as shown in FIG. 11, or both.

For example, although a light source consisting of one or more LEDs in aplane has been described, other forms of light source, including lightsources hereafter to be invented or developed, may be used. However, ifthe light source is not Lambertian, the shape of the lens and thereflecting back surface for a given beam pattern may be different. Inthe interests of simplicity, the LED has been approximated to a pointsource. Those skilled in the art will understand how a light source ofnon-negligible size will affect the shapes of the lens and reflector,and may limit the attainable precision of the beam pattern.

For example, although in all the embodiments the lens 202, etc. is asingle optical surface producing a single beam, the lens may, like thereflector 602, 613 or 1001, 1002, be divided into two or more sectionsproducing distinct beams.

The preceding description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles of theinvention. The full scope of the invention should be determined withreference to the Claims.

1. A collimator comprising a transparent body having: a central axis; acentral cavity bounded by a circumferential surface and a front lenssurface, the lens surface shaped to refract light from a predeterminedlocation on the central axis in the cavity through the body to and outthrough a front exit surface; an outer reflector surface shaped toreflect light from the predetermined location refracted by thecircumferential surface of the cavity through the body to and outthrough the front surface; wherein at least part of the reflectorsurface is not symmetrical about the central axis; and wherein thereflector and lens surfaces are arranged to direct the light in twobeams, one of said beams on said central axis and the other of saidbeams off-axis relative to said central axis.
 2. A collimator accordingto claim 1, wherein the circumferential surface of the cavity is asurface of rotation about the central axis.
 3. A collimator according toclaim 1, wherein the off-axis beam is narrower than the on-axis beam. 4.A collimator according to claim 1, wherein the off-axis beam is widerthan the on-axis beam.
 5. A collimator according to claim 1 thatprovides an on-axis beam from the lens surface.
 6. A collimatoraccording to claim 1 that provides an off-axis beam from the lenssurface.
 7. A collimator according to claim 1 that provides an on-axisbeam from a part of the reflector surface other than the said part.
 8. Acollimator according to claim 1 that provides an on-axis beam from afirst part of the reflector surface and an off-axis beam from a secondpart of the reflector surface.
 9. A collimator according to claim 1,wherein one of said first and second parts of said reflector surface isbetween the other of said first and second parts of said reflector andthe front exit surface.
 10. A collimator according to claim 1, whereinsaid reflector surface comprises at least two parts on different sidesof said central axis that are not symmetrical with each other underrotation about said central axis.
 11. A collimator according to claim 1,wherein the reflector surface is totally internally reflecting for lightfrom the predetermined location.
 12. A collimator according to claim 1,further comprising a source of light at the predetermined location. 13.A collimator according to claim 12, wherein the source of light isLambertian, and is coaxial with the central axis.
 14. A collimatorcomprising a modified photon funnel with means to provide off-axis ornon rotationally symmetric illumination where at least a section or theentire back TIR reflecting surface is modified from the standardon-axis, rotationally symmetric design.
 15. A collimator of the photonfunnel type that provides an off-axis narrow beam from the center lensand on-axis wide beam from the back surface.
 16. A collimator of thephoton funnel type that provides an on-axis narrow beam from the centerlens and off-axis wide beam from the back surface.
 17. A collimator ofthe photon funnel type that provides an on-axis wide beam from thecenter lens and an off-axis narrow beam from the back surface.
 18. Acollimator of the photon funnel type that provides both on and off-axis(either narrow or wide beam) light from dedicated sections from the backsurface.